1. Field of the Invention
This invention is directed toward logging of earth formations penetrated by a borehole, and more particularly directed toward the determination of formation density, formation porosity and formation fluid saturation from measures of fast neutron and inelastic scatter gamma radiation induced by a pulsed source of fast neutrons. A two-group diffusion model is employed which defines a distributed source of gamma radiation within the formation resulting from fast neutrons impinging upon and inelastically scattering from nuclei within the formation. Ambiguities in prior art measurements are removed by using iterative solutions of the inverted two-group diffusion model.
2. Background of the Art
Density logging systems, which are compensated somewhat for the effects of the borehole, were introduced in the mid 1960s in the paper “The Physical Foundation of Formation Density Logging (Gamma-Gamma)”, J. Tittman and J. S. Wahl, Geophysics, Vol. 30, p. 284, 1965. The system introduced by Tittman et. al., commonly referred to as a compensated gamma-gamma density logging system, was designed to operate in boreholes which are “open” and contain no steel casing. An instrument or “tool” is lowered into the well borehole on a cable, and the depth of the tool is determined by the amount of cable deployed at the surface of the earth. This type of tool contains an intense gamma-ray point source and preferably two gamma-ray detectors at differing distances from the source. The gamma ray detectors measure gamma rays that are scattered from electrons in the formation, and back into the borehole. Since for most earth formations, the electron density is in constant proportion to mass bulk density, the “backscatter” gamma ray intensity at the detectors provides a measure of formation bulk density. Two detectors at different axial spacings are preferably employed to allow the measurement to be compensated for the effect of mudcake that tends to accumulate on the borehole wall from drilling fluid used in the drilling process. The gamma-gamma density tool has a characteristic shallow depth of investigation into the formation of about 4 inches (“Depth of Investigation of Neutrons and Density Sondes for 35% Porosity Sand”, H. Sherman and S. Locke, Proc. 16th Annu. SPWLA Symp., Paper Q, 1975) and therefore is heavily influenced by the near borehole environment. This tool cannot make quantitative density logs in boreholes that have been cased, where the casing is typically steel and is surrounded by a cement sheath.
One technique for measuring formation porosity utilizes a porosity sensitive tool known in the industry as a “neutron-neutron” porosity system (Dual-Spaced Neutron Logging for Porosity”, L. Allen, C. Tittle, W. Mills and R. Caldwell, Geophysics Vol. 32, pp. 60-68, 1967). A two-group neutron diffusion model is used in developing the response of the logging system. The first group is an epithermal neutron group, and the second group is a thermal neutron group. The downhole tool portion of the system contains a source of fast neutrons, which is typically an isotopic source such as Americium—Beryllium (AmBe). At least two detectors sensitive to thermal or epithermal neutrons are axially spaced from the source at different distances. The detectors respond primarily to thermal or epithermal neutrons backscattered into the borehole by the formation. The measured backscatter flux is, in turn, primarily a function of the hydrogen content of the formation. If it is assumed that most hydrogen within the formation is contained in water or hydrocarbon in the pore space, the detectors respond to the porosity of the formation. As with the compensated density tool, the at least two neutron detectors respond to events at differing radial depths in the formation. The ratio of the detector response is formed to minimize the effects of reactions within the borehole, and porosity is determined from this ratio. The radial depth of investigation is about 9 or 10 inches, and the system can be calibrated to operate in both open and cased boreholes. It is again emphasized that this prior art methodology requires at least two detectors that are axially spaced from the neutron source.
U.S. Pat. No. 5,900,267 to Richard C. Odom et. al. discloses apparatus and methods for measuring the density of formations penetrated by a cased borehole. The borehole apparatus comprises a pulsed, fast neutron source and two axially spaced gamma ray detectors. The gamma ray detectors are biased to measure inelastic scatter gamma radiation produced by fast neutrons interacting with nuclei within the formation. Inelastic diffusion length LRHO is determined from the ratio of responses of the two detectors, and the measure of LRHO is subsequently related to formation bulk density. The physics of the LRHO bulk density logging technique is discussed in detail in the publication “Applications and Derivation of a New Cased Hole Density Porosity in Shaly Sand”, Richard C. Odom et. al., Paper SPE 38699, Annu. Technical Conference and Exhibition, 1997. Although operable in cased boreholes where previously discussed gamma-gamma density systems are insensitive, the LRHO system yields ambiguous results in formations saturated with liquid and saturated with a gas/liquid fluid mixture.
Prior art techniques for determining gas saturation of formations penetrated by an open borehole involve the combination of the responses of the conventional gamma-gamma type density tool and porosity sensitive neutron-neutron tool. When the density and porosity tools are calibrated for the water-saturated pore space condition, and when they log formations that are water-saturated, they will produce values for formation bulk density and formation porosity that are consistent with each other, assuming the tools are logging in a rock matrix that is the same as that used for calibration. However, when a formation zone is encountered where the pore water is replaced by gas, the porosity tool gives an erroneously low porosity indication, while the density tool correctly indicates a decrease in bulk density with corresponding apparent increase in porosity. This results in a “cross-over” of the log response curves from the two tools thereby indicating the presence of gas within the logged formation. This method is problematic in cased boreholes because of the more shallow investigation depth of the density log and its resulting greater sensitivity to variations in borehole conditions, such as variations in the thickness of the cement sheath, immediately behind the casing.
Logging for gas in cased, cemented boreholes has been performed using a logging tool containing a pulsed source of fast neutrons. Details of this method may be found in “Examples of Dual Spacing Thermal Neutron Decay Time Logs in Texas Coast Oil and Gas Reservoirs”, Trans. SPWLA 15th Annu. Logging Symp., 1979, and “The Use and Validation of Pulsed Neutron Surveys in Current Drilling Tests” Trans. SPWLA 19th Annu. Logging Symp., 1978. This “pulsed-neutron decay time” or “pulsed neutron” tool, as it is known in the art, was designed to detect the presence of hydrocarbon liquids (oil) in formations where the water that otherwise fills the pore spaces is normally saline. This sensitivity to fluid type is achieved by measuring the formation thermal neutron cross section. However, the cross section is not very sensitive to the presence of gas, and therefore the logging tool is not very useful as a gas indicator. Another type of measurement can be performed with the pulsed-neutron decay time tool that is more sensitive to the presence of gas. This involves measuring a ratio of the tool's typically two axially spaced gamma detector responses. This ratio can, in turn, be interpreted in a manner that is sensitive to the presence of gas within the logged formation. Since the measured gamma radiation is produced by thermal neutron capture reactions, this response is similar to that of the neutron-neutron porosity log in that both are responding to changes in the spatial distribution of thermal neutrons which, in turn, is a function of hydrogen density. For this reason, the gamma ratio response, like the neutron-neutron porosity tool response, is not capable of distinguishing between low porosity formations and formations with higher porosity whose pore spaces are gas filled. The density/gas saturation logging system set forth in this disclosure, on the other hand, responds to the change in atom density and hence can distinguish between the gas saturated high porosity and liquid saturated low porosity formations.
U.S. Pat. No. 5,804,820 to Michael L Evans et. al. discloses a density logging system that uses a source of fast neutrons, and a single gamma ray detector. A measure of fast neutron attenuation is required to obtain a meaningful density measurement from the response of the single gamma ray detector. A neutron detector is used to monitor source neutron attenuation. Alternately, it is taught that fast neutron attenuation can be inferred by making use of the fact that neutrons interacting with the various components of the tool result in the production of both inelastic and epithermal capture gamma rays. For example, fast neutrons interacting with the iron of the tool yield inelastic gamma radiation. A measure of intensity of this gamma radiation can be used to infer fast neutron attenuation. Gamma radiations from iron are quite prominent in the inelastic gamma-ray spectrum and as a result, are easy to use without complicated spectral deconvolution techniques. There is no teaching of methods to resolve ambiguities in density measurements for formations saturated with liquid and formations saturated with a significant fraction of gas.
U.S. Pat. No. 6,207,953 to Robert D. Wilson discloses a logging system that utilizes a source of fast neutrons. The system combines a measure of fast neutrons along with measures of inelastic gamma radiation at two axial spacings from the source to provide data used to compute formation porosity and liquid saturation. The computation is based on an interpretation chart that plots fast neutron count rate against an inelastic gamma ray ratio measured at the two axial spacings. Formation density can be computed from formation porosity if matrix and fluid densities are known. Inelastic diffusion length LRHO is a function of the measured ratio. A liquid scintillator is specified for fast neutron detection, providing both fast neutron and inelastic gamma counts by pulse-shape discrimination. An alternate plastic scintillator and gamma detector combination is also taught in the event that the liquid scintillator is not suitable for a particular application. Fast neutron energies are distinguished by use of pulse-height discrimination to provide borehole size compensation for air-filled boreholes. Although the system does minimize ambiguity in liquid and gas filled formations, there are still deficiencies in the methodology. The axial spacing of the neutron detector from the source is not optimized to minimize adverse borehole effects because of count rate statistics limitations. Inelastic scatter diffusion is determined from the ratio of inelastic scatter radiation at two axial spacings from the source. This methodology essentially assumes a point gamma radiation source, which physically is not robust as will be discussed in subsequent sections of this disclosure.